The study of interconnected neural systems is crucial to understanding neural function. Until recently, attempts to measure electrical activity in neural networks have been pursued using either multiple single micropipettes, multibarrelled micropipettes, or single electrodes which record simultaneously from multiple sources of activity. However, these methods are unsuitable for recording from large numbers of neurons simultaneously.
For approximately the past twenty years, sporatic efforts have been undertaken to use integrated technology to record from collections of neurons. An example of such an effort is described in "An integrated-circuit approach to extracellular microelectrodes," by K. D. Wise et al, in IEEE Transactions on Biomedical Engineering, Vol. 17, July 1970. However, technical achievements in electrode fabrication technology have not been used extensively in the biological applications area. One reason for this is that high-technology electrode designs and fabrication techniques are generally unavailable outside of a small group of original developers. Furthermore, probe chips for biological applications are generally not designed to be made in commercial silicon foundries for reasons related to mechanics (e.g. pointed probes) and tissue compatibility (e.g. special materials).
One prior approach to making microelectrode arrays has been to use thin-film microlithography techniques to deposit gold electrodes on a glass substrate, as described in "Recording action potentials from cultured neurons with extracellular microcircuit electrodes," by J. Pine, in Journal of Neuroscience Methods, Vol. 2, 1980, and in "Recording from the Aplysia abdominal ganglion with a planar microelectrode array," by J. L. Novak and B. C. Wheeler, in IEEE Transactions on Biomedical Engineering, Vol. 33, February 1986. Resulting electrode dimensions have been on the order of 10-25 microns. Another approach has employed special methods adapted from integrated-circuit fabrication techniques, as described in "A high-yield IC-compatible multichannel recording array," by K. Najafi et al, in IEEE Transactions on Electron Devices, Vol. 32, July 1985. In this probe, an array of gold electrodes was supported on a silicon carrier. By selectively removing an insulating layer of silicon dioxide which covers the probe, the effective electrode-tip diameters were made as small as 2 microns in diameter. The design was compatible with incorporation of on-chip circuitry.
In these prior approaches, the issue of biocompatibility has been an important consideration, leading to use of special materials (gold for example). For cases where biocompatibility is not an important issue, such as in short-term experiments, there is a need for a cheap standard microelectrode array microchip. To meet these goals, a design is required that can be made on a commercial fabrication line.